What do you do when you want to run your Intel i7 860 at 4GHz with air cooling? My approach was to get a Prolimatech Megahalems. I bought some Noctua low noise fans. I’m set, right? I can run my computer in my non-airconditioned computer room and use it all summer, right?

Wrong. It turns out that I had bitten off a bit of a challenge. I was going to need some stronger fans. But to choose fans I couldn’t just rely on fan specs. Many companies selling items to consumers present specifications that are not always accurate (I’m being charitable here). I couldn’t just read reviews, since they were either oriented to radiators (the Megahalems is not a radiator) or to case fan use, or used methodology that was not helpful to me. Besides, how do airflow (in CFM) or static pressure relate to the cooling power of a Megahalems? I could just buy a real brute of a fan, but the computer must sit on my desk not far from my ears. I wanted a balance between performance and noise.

I decided to test a lot of fans to see what they could do to cool my system down. Since the Megahalems is designed not to put up much resistance to airflow, I decided that I would also use it as a proxy for testing the airflow of various fans so I could determine which fans I would prefer as case fans. So I tested a number of fans that ranged in CFM. In the end, I tested 65 fans in 112 configurations. I tested them in forty-minute runs, sometimes more than once.

Limits

I don’t want to cook my chip. Yes, I know they can handle lots of heat, but I decided I would stay within official parameters. That meant, for example, that I don’t want to let the temp at the center of the heatspreader – Intel’s official measuring point – exceed the “thermal design power” of the chip, which is 72.7c. Now, I do get a reading from my motherboard that seems to be the heatspreader temp (HST). The motherboard software calls it the “system temperature.” But I am not absolutely sure that the system temperature is the HST. In any case, my hottest cpu core temp consistently runs 9c over the system temp. Also, because the cpu temps are higher they are more accurate because the chip reports not the actual temperature but the distance in centigrade from the maximum allowed junction temperature – the chip will throttle itself above its Tjmax of 99-100c – and the smaller the distance from the reported temp to Tjmax, the more accurate it is. Finally, when people report temps they report cpu core temps. So I will report cpu temps, and the max cpu temp I will allow is 80c. I could push that to 81.7c but I don’t trust the accuracy of the sensors. I am also concerned about heat transfer.

Working backwards, then, the max allowable difference in temperature over ambient will be 50c. This gives me a rough measure that I can transfer from my basement workshop where the ambient is 16-19c to our computer room where the ambient can reach 30c. I call it a rough measure because the higher the ambient, the less the capacity of a heat source to transfer heat to a heatsink. Because I do not know what the magnitude of this difference is, I am keeping a max heat difference of 50c and not 51.7c.

The Setting and the Setup

My Basement workshop is cool and quiet, with ambient temps ranging from 16c to 19c, and ambient noise less than 10 dBA when there is no activity upstairs.

The chip and motherboard are set in an NZXT Beta Evo that has had its top and back grills removed, and set on its side. The Megahlems was oriented front to back (or right to left, if you will). The exhaust from the heatsink blew right out the back where the rear grill used to be. The case was not moved for the duration of the testing, and the side panel was left off. No case fans operated during the testing.

TIM was Gelid Solutions GC Extreme (GC-3) applied as a double grain of rice in the center of the heatsink. The video card is a low performance ( = cheap) fanless card.

The fans were connected to the motherboard CPU fan header using a fan Y-type cable splitter. The yellow RPM reporter wire on the exhaust fan was split off from the Y-cable and connected to another fan header on the motherboard so that RPM’s from each fan could be monitored. However, with 5-volt testing I rigged a pair of Molex adapters to provide 5v power to the fans while allowing RPM monitoring.

The reason for connecting the fans to the motherboard cpu fan header is that during normal use each heatsink fan (HSF) will be on Auto – controlled by the motherboard to respond to cpu temps. During idle there is no need for a fan to be going flat out.

Speaking of not going flat out, you may well ask why I tested fans at 5 volts. I was trying to get some idea of how loud these fans would be, and how well they would perform undervolted. Why not use a fan controller? Some fan controllers make fans sound bad when they are undervolted. So I simply used the 5v line from Molex to test fans at the lower voltage. This way we get an honest result, untainted by a less than stellar controller.

The Overclock

The i7 860 I am using has reached a BCLK of 230MHz on 1.396v. This was with the motherboard on Auto. The max for the chip is 1.4v. The motherboard has been reliable: it has never let the voltage go over 1.396v on Auto. This chip has also gone to 4.5GHz at BCLK 207MHz. But I did not and do not have cooling systems to allow me to stress test my system at those speeds and voltages.

For a 4GHz overclock, the user of an i7 860 has a few choices: a 200MHz BCLK with a 20x multiplier (= 4000MHz), 191 x 21 = 4011MHz, and 182 x 22 = 4004MHz. Higher BCLK’s require higher voltages, so the 182MHz BCLK was the obvious choice. Unfortunately, the chip won’t allow higher multipliers so a lower BCLK cannot be used.

The motherboard sets voltages in increments. The lowest setting that would allow a twelve hour run of LinPack was 1.31250v. With Load Line Calibration enabled, the cores get up to 1.328v under max load. Vtt is 1.19v (absolute max allowed is 1.21v). Vdimm is set to 1.620v and runs at 1.616v. Oh, yes: hyperthreading is enabled.

As for stability: on top of the twelve hour OC stability runs, the rig has done at least 150 forty-minute fan test runs of OCCT/LinPack with exactly zero errors.

The Instruments

For ambient temps I have a digital thermometer. It measures in Fahrenheit, so I convert to centigrade and round to the nearest 0.5c. The basement is not only cool, but the ambient temp changes slowly. For cpu temps I use Real Temp to record max temps, and OCCT to graph the cpu temps. Both rely on what the motherboard (a Gigabyte GA-P55A-UD3P) reports from the cpu sensors.

For RPM’s I use EasyTune6, the app Gigabyte bundles with its motherboards. The RPM jumps around. Even on Speedfan that RPM number jumps around on all of these fans. So I try to assess the average speed during peak load. The RPM readings are the least accurate aspect of these tests.

Fan sound level was measured with the Tenma 72-942 sound pressure level (SPL) meter, which purports to be accurate to 0.5 dB at 30 dB and above. It will measure quieter sounds but does not promise such accuracy. So, for example, I was able to determine that my basement has an SPL of less than 10 dBA when all is quiet upstairs, but not a definite number. For the fan SPL I measured dBA at 10cm, then converted the reading to the equivalent of 1m by subtracting 20 dB. The check on this is that certain industrial fans like San Aces give accurate specifications. The SPL meter readings were either at or close to industrial specs in free air.

I expected that the fans would all be louder on the heatsink. Fans get louder when their output is obstructed and the air pressure rises. But with the Megahalems the fans frequently were not as loud as their specs. Sometimes this was due to lower RPM’s than spec, but other times – who knows?

Fan noise is not the same as SPL, though. The Magma and the Slip Stream, for example, put out a lot more sound pressure level than noise. The SPL meter clearly picks up all of the white noise that we simply do not perceive. Comments here are important.

The Load

After comparing LinX, Prime95 and OCCT, I found I was running my hottest temps on OCCT’s Linpack module. If I were testing for stability of overclock I’d run both Prime95 and OCCT. But I’m looking for heat stress here. I tested runs of up to two hours, but I discovered I got no more information than I had at 40 minutes.

Limitations of the Testing

The CPU reports temps in 1c increments. While the System Temp reading tended to stay stable at one reading or another, the cpu temps tended to jiggle up and down. When this happened a lot, it was fairly clear that the real temp was somewhere between the upper and the lower temp, so I recorded it as x.5c. The ambient I recorded to the nearest 0.5c. I recorded the difference between the hottest cpu temp and the ambient temp as ‘temp over ambient.’

Most fans had a single run. With the more important fans I did more than one run. Where I have more than one of a fan I will try to run each copy of the fan. The results are averaged, but generally temp over ambient ranged about 1c or less between runs.Edited by ehume - 5/4/10 at 4:49pm

When I started this investigation I had some notion that there would be a large spread in cooling prowess between fans. I thought that most of my fan testing would be at 3.6GHz, and that only the best fans would cool my system at 4GHz. So much for expectations.

Conclusions:

1. Once you get to 1000 RPM or so, there is only about a 12c difference in cooling performance between the various fans.

2. As a practical matter, fans seem to max out between 2000 and 2500 RPM. After that you need huge increases in CFM to get another degree of cooling.

3. Although putting on a second fan helps, the amount it helps depends on the power of the first fan: the more powerful the fan, the less benefit you get from a second fan.

4. TANSTAAFL: By and large, if you want more air through your cooler, you will have to pay for it with more noise.

5. The blade count doesnâ€™t seem to make much difference in cooling, but the depth of the fan does.

The San Ace 9G1212H101(1) remains the Prince of Fans, at least on the Megahalems. One of these gives the best balance between sound pressure level and cooling performance.

And the Princesses of Fans: A pair of Yate Loon D12SH-12â€™s gives nearly the same cooling power with slightly less noise; but at the low voltages seen at idle, they will be quieter. Also, at US$7.40 for a pair, they are far cheaper than the $18.40 a US vendor wants for a single San Ace with bare wires.

I got an Akasa Apache because I like to read about airplanes and Akasa’s pictures of this fan – taken from the side – made the blades look like the blades of a large jet engine compressor. Neat! I thought.

In my opinion this fan would do better as a case fan than a HSF. More recently Akasa has released a PWM fan with an RPM spread better suited to overclocking. The Viper fan ranges from 600 to 1900 RPM. If you’re thinking of getting an Apache for a HSF, I’d recommend a Viper instead.

This fan came with my case. It comes with a Molex connector for power and has a three-way switch for speed control. It does not have an RPM reporting lead, so speeds must be estimated. I synchronized another fan with it, then read the RPM of that fan.

I got this fan as part of a deeply discounted random collection of fans. From looking at it you’d think it is only a case fan. Yet a 92mm version of this fan is the air mover for the AC Freezer Pro 7, an excellent little CPU cooler. One unusual aspect of this fan: when you look at it from the intake side the blades rotate clockwise. Also, the exhaust is the open side of this fan.

I wanted a shroud. That meant tearing the guts out of a fan. I found a fan discounted to $2US (normally $2.79). But it was too nice a fan to kill, so I kept it. When I got my grab bag (a deeply discounted random collection of fans) it contained some useless fans which are now shrouds, thus preserving this “bulk” fan.